1. Field of the Invention
This invention relates to an ink-jet printing method and, more particularly, it relates to an ink-jet printing method for printing characters and images on a sheet of printing paper, OHP paper or the like, using an ink containing a coloring material and a liquid for insolubilizing the coloring material contained in the ink (to be referred to as treatment liquid hereinafter).
2. Related Background Art
The ink-jet printing system has various advantages including a low noise emission level, low running cost, the potential for high speed printing and down-sizing, the ease of color printing and so on and hence It is currently widely used in various printers and copying machines. With a printer using an ink-jet printing system, ink is generally selected from the viewpoint of ink ejection performance, printing characteristics including fixability and printing quality including blurring of printed images, optical reflection density and color development properties. Meanwhile, it is well known that there are two types of ink in terms of the coloring material contained therein. They are dye ink and pigment ink.
Pigment ink is advantageous to dye ink from the viewpoint of water-resistance and light-resistance and can print characters clearly to achieve high quality printing. On the other hand, pigment ink takes more time than dye ink for fixation on a printing medium and is not satisfactory in terms of abrasion resistance of the image after fixation if compared with dye ink. Additionally, each ink dot of pigment ink formed on a printing medium by a single ink-ejecting operation of a nozzle is relatively small when compared with that of dye ink. The pigment contained in the pigment ink can be reliably and stably dispersed in the ink by means of the electric repulsion of a polymeric dispersant so as to overcome the intermolecular force which acts between the pigment particles to cause aggregation. Therefore, it is preferable to add a polymeric dispersant to pigment ink depends on the pigment content of pigment ink.
When a sheet of plain paper is printed with pigment ink by means of an ink-jet recording method, the pigment particles contained in the ink aggregate as the solvent of ink, such as water, penetrates into the paper and evaporates into the air. Then, in terms of the behavior of the ink on the sheet of paper, the aggregational force of the ink becomes stronger when the polymeric dispersant is contained in the ink in a higher content. As a result, each ink dot formed on the printing medium by an ink having a predetermined volume ejected from an ink-jet head tends to minimize its diameter and keep a distorted profile produced as a result of the collision of the ink with the surf ace of the sheet of paper. Therefore, in order to obtain ink dots having a recording density necessary for forming an image on the sheet of paper and having a dot diameter necessary for forming an image without generating white stripes, some regulating measures should be taken to have a rather large volume of ink ejected from the ink-jet head. However, even with such regulating measures, the fixation of ink to the printing medium can take a long time and the scratch resistance of the recorded image can be reduced, because of the strong aggregational force of the pigment particles adsorbed by the polymeric dispersant and the resultant poor permeation of ink into the sheet of paper.
The use of a penetrant may be conceivable to improve the ink permeability of the printing medium in order to increase the ink dot diameter and improve the fixation of ink to the printing medium. However, the use of a penetrant can be accompanied by a problem of a degraded ink dot profile (a degraded circumferential profile of each ink dot such as feathering) and that of permeation of ink to the rear surface of the sheet of paper (so-called back-through) that are undesirable for producing high quality recorded images. Additionally, if the ink dot diameter is relatively large, the OD (optical density) of the ink dot may not be raised because the coloring material of the ink can penetrate into the printing medium. Still additionally, there is a problem that a higher printing speed will be required to ink-jet printers in business applications where the demand for ink-jet printers is expected to rise dramatically. If fixation of ink to the recording medium is not enough in a process of successively discharging printed sheets of paper from an ink-jet printer and sequentially laying them one on the other, a second sheet of paper discharged from the ink-jet printer may be laid on the first sheet of paper while the ink on the surface of the first sheet of paper is not fixed enough, so that there may arise a problem that the image on the first sheet of paper becomes blurred and/or the ink on the first sheet of paper sticks to the rear surface of the second sheet of paper.
Meanwhile, an ink containing a self-dispersion type pigment has been proposed. With such ink, the ink dots formed on a sheet of paper give an increased dot diameter probably because the pigment contained in the ink has an aggregational force weaker than that of ordinary ink pigment that is dispersed by a polymeric dispersant, when applied onto the surface of paper. However, the increased dot diameter is still not satisfactory.
As pointed out above, the currently available ink-jet printing methods still have much room for improvement in various aspects relating to the quality of the recorded image that include fixation of ink, enlargement of ink dot diameter, uniformity of density of each ink dot and high optical density of ink dots.
On the other hand, in the field of ink-jet printing technology, methods of applying ink and treatment liquid that reacts with the applied ink have been proposed In order to improve the quality of printed characters and images (e.g., in terms of water resistance and optical density of the image formed on a printing medium) by causing the applied ink and the treatment liquid to favorably react each other on the printing medium. Some of such methods have found practical applications.
Under the above identified circumstances, the inventors of the present invention have paid intensive research efforts for developing the ink-jet recording technology of using in combination a pigment ink and a treatment liquid that destroys the dispersibility of the pigment ink at the time of recording. In the course of the researches, the inventors experimented a recording process of applying a treatment liquid onto the surface of a printing medium and subsequently applying a pigment ink in such a way that the latter is mixed with the treatment liquid in a liquid state on the printing medium. As a result, it was found that some of the obtained images showed an image quality that is far from satisfactory and is sometimes even worse than the quality of images formed by using only pigment ink. More specifically, in the case of a combination of a pigment ink containing a pigment dispersed in an aqueous medium by means of a polymeric dispersant and a treatment liquid capable of reacting with such ink, the produced ink dots had a low optical density (OD) probably because of a small area factor of the ink dots. Although why such a phenomenon occurred is not clear, the inventors presume that the aggregation of pigment in the ink was greatly accelerated by the treatment liquid on the printing medium.
The area factor may be raised to enhance the OD by increasing the shot-in quantity of the pigment ink. Then, however, the fixation of ink can sometimes be worsened. In the case of a combination of a pigment ink containing a self-dispersion type pigment and a treatment liquid capable of reacting with such pigment ink, the dots formed on the printing medium showed the phenomenon of so-called exudation or haze along the circumferences thereof to blur the profiles of the dots. FIG. 1 of the accompanying drawings schematically illustrates a dot showing such exudation or haze As shown in FIG. 1, the phenomenon of exudation or haze can be observed along the reaction zone of a pigment ink 8 that is located at the center and a surrounding treatment liquid 6. FIGS. 2A through 2C of the accompanying drawings schematically illustrate the presumed mechanism of appearance of the phenomenon.
After a treatment liquid S is applied onto a printing medium P (a sheet of plain paper in particular), a pigment ink Ip containing a self-dispersion type pigment but not containing any polymeric dispersant is applied onto the treatment liquid S. Then, the treatment liquid S and the pigment ink Ip react each other to start producing a reaction product 9 (see FIG. 2B). As the reaction proceeds, exudation of the reaction product appears radially from the circular dot of the ink as shown in FIG. 2C to make the entire dot appear as if it were surrounded by haze. Such exudation or haze is apparently recognized as feathering that is a known phenomenon to consequently degrade the quality of printing.
The Inventors of the present invention assume that such exudation or haze is a phenomenon that can be explained in a manner as described below in chemical or micro-dimensional terms. The pigment ink containing no dispersant reacts with the treatment liquid at a relatively high reaction rate so that some of the dispersed pigment particles are instantaneously subjected to dispersive destruction to produce clusters of the reaction product together with micro-particles of the reaction product. Then, the particles of the reaction product start flowing out as the permeation front SP in the printing medium expands, and thereby the exudation or haze appears.
As described above, a situation unpredictable to the inventors of the present occurred when a pigment ink and a treatment liquid were simply combined, so that it was difficult to obtain a high quality image by ink-jet recording. As a result, the inventors recognized that further efforts were required for technological development in order to achieve the initial object of eliminating the drawbacks of pigment ink, while utilizing the ink-jet recording process using a treatment liquid and exploiting the advantages of a pigment ink.
Furthermore, as pointed out above, a higher printing speed will be required to ink-jet printers in business applications where the demand for ink-jet printers is expected to rise dramatically. One of the major problems of high speed printers is the fixation of ink on a recording medium. If fixation of ink to the recording medium is not satisfactory in a process of successively discharging printed sheets of paper from an ink-jet printer and sequentially laying them one on the other, a second sheet of paper discharged from the ink-jet printer may be laid on the first sheet of paper while the ink on the surface of the first sheet of paper is not fixed enough, so that there may arise a problem that the image on the first sheet of paper becomes blurred and/or the ink on the first sheet of paper sticks to the rear surface of the second sheet of paper to consequently damage the appearance of the printed sheets of paper.
Thus, the present invention is based on the technological findings of the inventors of the present invention. It is an object of the present invention to provide an ink-jet printing method to produce higher quality printed images by utilizing the ink-jet recording technology using a pigment ink and a treatment liquid.
Another object of the present invention is to provide an ink-jet printing method which can achieve high speed fixation of ink to the recording medium without sacrificing the quality of the printed image.
In an aspect of the invention, the above objects are achieved by providing an ink-jet printing method to be used for a process of recording an image on a printing medium, comprising:
a first step of applying an ink onto the printing medium by an ink-jet recording method; and
a second step of applying a treatment liquid containing at least one of a polyvalent metal ion and a salt thereof;
said first step being conducted subsequent to said second step so as to cause the ink and the treatment liquid to come into contact with each other in a liquid state on the printing medium;
said ink containing a first pigment, a second pigment and a polymeric dispersant for dispersing said second pigment in an aqueous medium, said first pigment and said second pigment being contained in said ink in a dispersed state;
said first pigment being a self-dispersing pigment having at least one anionic group bound to the surface of said first pigment directly or by way of another atomic group, said second pigment being capable of dispersing-in said aqueous medium by means of said polymeric dispersant;
said polymeric dispersant containing at least one of a polymeric dispersant having the same polarity as that of the group bound to the surface of said first pigment and a nonionic polymeric dispersant;
said treatment liquid containing at least one of a polyvalent metal ion and a salt thereof, and destabilizing dispersion stability of at least one of said pigments contained in said ink when the treatment liquid and the ink are applied onto the printing medium so as to come into contact with each other in a liquid state.
In another aspect of the invention, there is also provided an ink-jet printing method comprising a step of applying a first ink, a second ink and a treatment liquid so as to come into contact with each other in a liquid state on the surface of a printing medium;
said first ink containing as a first pigment a self-dipersing pigment having at least one anionic group coupled to the surface thereof directly or by way of another atomic group;
said second ink containing a second pigment and a polymeric dispersant for dispersing said second pigment in an aqueous medium, said second pigment being capable of dispersing in said aqueous medium by means of said polymeric dispersant, said polymeric dispersant containing at least one of a polymeric dispersant having the same polarity as that of the group coupled to the surface of said first pigment and a nonionic polymeric dispersant;
said treatment liquid containing at least one of a polyvalent metal ion and a salt thereof and destabilizing dispersion stability of at least one of said pigments contained in said inks when the treatment liquid and either of the inks are applied onto the printing medium so as to come into contact with each other in a liquid state, said treatment liquid being applied onto said printing medium prior to the application of said first ink and said second ink.
The above methods can provide a high quality image having a large area factor and a high OD value and being significantly free from haze. Additionally, the obtained image is excellent in terms of scratch resistance, fixation and various other respects.
While the reasons of the above advantages of the methods according to the invention are not clear, the inventors have confirmed the following facts as a result of a series of experiments carried out for the purpose of the present invention.
Firstly, when an ink containing a first pigment and a second pigment is applied to an area of a printing medium where a treatment liquid containing a polyvalent metal ion or a salt thereof has been applied, in such a way that the ink is overlapped the treatment liquid in a liquid state or come into contact with the latter, the ink dot in the area where the treatment liquid has been applied, expand considerably to produce an ink dot having a large diameter.
A technique of suppressing the phenomenon of mixed colors (bleeding) along the boundary lines of different colors of an image by applying a pigment ink of a polymeric dispersion type and an ink containing a polyvalent metal salt so as to bring them into contact with each other on a printing medium to destabilize the dispersion stability of the pigment is disclosed in a number of patent documents including U.S. Pat. No. 5,518,534. However, as a result of a number of experiments conducted by the inventors of the present invention, it was impossible for any combinations of inks described in the disclosed patent documents to produce ink dots having a large diameter that can be obtained by the above described methods of the invention.
On the basis of the above findings, the inventors believe that the effect of aggregation of a polyvalent metal ion and each of the pigments is alleviated as a result of the coexistence of the first pigment, the second pigment and the treatment liquid containing ions of the polyvalent metal or a salt thereof, although each of the pigments and ions of the polyvalent metal react with each other to produce an aggregated product.
More specifically, the aggregation caused by the reaction of the first pigment and the polyvalent metal ion and the reaction caused by the second pigment (including the polymeric dispersant) and the polyvalent metal ion differ from each other in terms of strength, so that the effect of aggregation of the stronger reaction is alleviated by that of the weaker reaction and additionally that the strong intermolecular force of the second pigment in the reactive solution Is alleviated by the presence of the first pigment As a result, the ink tends to spread horizontally on a sheet of paper.
In the case that the treatment liquid containing a polyvalent metal ion or a salt thereof reacts with the first pigment, haze can scarcely be observed. Thus, in spite of the increase of the dot diameter, the produced image shows sharp edges without haze or the like.
In addition, the ink shows excellent fixing properties since a large dot diameter can be obtained with a small amount of ink, as Pointed out above. Furthermore since the use of the first pigment makes it possible to reduce the amount of the polymeric dispersant in the ink, fixing properties are far more improved.
The fixing properties and the dot diameter are further improved when using a treatment liquid which can penetrate into the printing medium at an enhanced rate. This is because the treatment liquid that can penetrate into the printing medium is diffused quickly into the printing medium, and a sort of an ink receiving layer is foamed on the surface of the printing medium. Consequently the ink can easily permeate and spread along the surface of the printing medium, and forms dots while reacting with the treatment liquid, to form a large dot quickly on the surface of the printing medium.
Furthermore, in order to further improve the image quality, it is preferable to optimize the concentration of the polyvalent metal ion or a salt thereof in the treatment liquid and the amount of the treatment liquid applied on the printing medium.
The OD of the image formed on the printing medium is satisfactorily high when the concentration of a polyvalent metal ion or a salt thereof is about ⅓ of that of the pigment in the ink. It is not necessary to make the concentration excessively high.
The fixation of the ink is improved when the concentration of a polyvalent metal ion or a salt thereof is lower than the concentration of the pigment. Additionally, the treatment liquid may preferably be applied in an amount of xe2x85x9 to xc2xd of the amount of application of the ink since the OD and the edge sharpness of the formed image are improved.
With the above arrangement, it is possible to form a high quality image with a high OD value and excellent fixing properties with a short fixing time on a printing medium without giving rise to any haze.